A various types of optical devices allowing a viewer to perceive a virtual image superimposed on an image in real space have been recently developed. An example of such optical devices includes a wearable optical device, and in particular, a device that is worn by the user on the head is known as a head-mounted display (HMD). An exemplary type of the HMD is known in which a half mirror serving as a display surface is provided in front of the viewer's pupils and an image (real image) is formed on the display surface. Another type of the HMD is developed in which guidance of image display light to the viewer's pupils using an optical system allows the viewer to perceive an image (virtual image). As one example, Patent Literature 1 discloses the technique that implements an HMD capable of guiding the image display light in the lateral direction with respect to the viewer's pupils to be incident on the viewer's pupils.
A technique for enhancing visibility of an image even if brightness of real space varies in the case where an image in a real space and a virtual image are viewed using such an HMD has been also developed. As one example, Patent Literature 2 discloses the technique that detects illuminance of the light passing through an optical system of the HMD from the front of the viewer's eyes and is guided to the viewer's eyes. In addition, this technique gradually decreases the luminance of image display light when illuminance changes from a bright region to a darker one, and gradually increases the luminance of image display light when illuminance changes from a dark region to a brighter one. This technique allows satisfactory visual recognition of an image to be achieved even when the illuminance of the light, which passes through an optical system from the front of the viewer's eyes and is guided to the viewer's eyes, is very low or high.